It is desirable that cameras used for photographing action events be as automatic as possible in order to free the cameraman to concentrate on picture composition. Ideally, the cameraman should only have to concern himself with the subjective element of framing (pointing and zooming) and not the more objective elements of focusing and exposure.
Focusing is not a significant problem at events which occur in daylight since the lens aperture may be reduced to the point where the depth of field is adequate. Focusing is a significant problem, however, at indoor or night events where low light levels necessitate using large lens apertures, particularly if color fidelity is to be maintained. Such apertures produce shallow depths-of-field and necessitate careful and frequent refocusing. As the lens is zoomed to increase image size the focusing becomes even more critical. A further problem is that when an image is slightly out of focus, it is difficult to determine whether focusing is long or short without a trial-and-error procedure. Thus, a cameraman usually experiences difficulty shooting an action sequence under artificial lighting conditions with tight framing and acceptable maintenance of focus.
Focusing aids such as split-image and microprism rangefinders are seldom employed in action photography because they are too slow. A partial solution to the problem is to establish focus presettings which may be selected by buttons or stops associated with certain frequently used distance settings. One button could be preset, for example, to the distance to a hockey goal. Focus presets were developed, however, for use in staged studio applications where subjects are located at only a few specific fixed distances, and are of little use in sports photography where sharp focusing is required over a continuum of focal distances.
Autofocusing devices for still cameras are disclosed in the literature (see, for example, U.S. Pat. Nos. 3,538,830 through 3,538,833; 3,572,230; and 3,730,068). Devices of this type commonly reference to a pendulum, and assume that the photographer is standing on the same baseline as the subject. They require that he momentarily point the camera at the subject's feet before each shot to achieve focus. This deframing procedure and the oscillations of the undamped pendulum induced by slewing the camera preclude its application in motion photography.
Other known autofocusing devices recognize that, for cameras situated outside the plane of the subject, focusing information can be derived from knowledge of the lens focal length, height of the camera, and angle by which the camera is pointed below the horizon (see, for example, U.S. Pat. No. 2,456,317). To solve this equation which has three variables, interchangeable three-dimensional cams are used. However, these cams are difficult to manufacture and are difficult to mount for proper tracking in the camera stand.